Atrazine mineralisation rates in New Zealand soils are affected by time since atrazine exposure

To understand more clearly the groundwater contamination potential of herbicides applied to New Zealand soils, experimental field plots were established on 2 different soil types: Himatangi, a sandy dune soil, and Kiripaka, a silty clay derived from basalt. A mix of triazine herbicides, containing atrazine, terbuthylazine, and hexazinone, was applied to the plots at 10 kg a.i./ha. At various times after application, soil was removed from the plots and analysed for residual levels of herbicides, in vitro rates of mineralisation of C-14-ring-labelled atrazine, and numbers of atrazine-degrading microbes. Atrazine and terbuthylazine were below detectable levels (<0.01 mg/kg) in Himatangi topsoil 18 months after pesticide application but still detectable in topsoil from the Kiripaka site. Hexazinone was detectable in topsoil from both soil plots 18 months after application. Atrazine adsorption isotherms were constructed for topsoil and subsoil from both plots, with estimated K-f values ranging from 0.53 to 4.69 mu g(1-n) mL(n)/g. A single application of atrazine was sufficient to enhance the rate of C-14-atrazine mineralisation in vitro by topsoil from both plots, and subsoil from the Kiripaka site. Rates of mineralisation of atrazine in the soil from the plots increased 1-6 months after pesticide application and remained elevated for 18-24 months. The numbers of atrazine degraders detected did not correlate with atrazine mineralisation rates. An atrazine-degrading bacterium, identifed as Arthrobacter nicotinovorans, was isolated from Himatangi soil exhibiting enhanced rates of atrazine-mineralisation activity.